There is a way to update a table, but I haven't ever done that, so I'm not familiar with it. Also your formula is not going to get you all of the data you're looking for.
Thinking about this formula, here's a way you can do it without using variables:
left({Test.Tag}, InStr({Test.Tag}, " ")) + {Test.Size} + " " + Right({Test.Tag}, (Length({Test.Tag} - InStr({Test.Tag}, ""))
You may have to "play" with the length in the "Right" statement - using "+ 1" or "-1" (I can never remember which without playing with it...) to start the second half of the string in the right place.
-Dell